Cement retarder systems, and retarded cement compositions

ABSTRACT

A cement retarder system for use in underground wells included a borate compound, an organophosphonate salt, and a copolymer formed from AMPS and a monomer selected from the group consisting of acrylic acid, acrylamide and mixtures thereof. A method of using the retarder system in an underground well having a borehole drilled therein with a borehole wall, and further including a casing disposed in said borehole, such that an annulus exists between said casing and said borehole wall, comprising the steps of providing a cement composition and a liquid carrier, providing a retarder system for said cement composition comprising a borate compound, an organophosphonate salt, and a copolymer formed from AMPS and a monomer selected from the group consisting of acrylic acid, acrylamide, and mixtures thereof, mixing a sufficient amount of said retarder system to create a desired induction period with said cement composition and said liquid carrier to form a cement slurry, pumping the cement slurry from the surface down the casing, and causing said slurry to return to the surface in the annulus between the casing and the borehole wall.

RELATED APPLICATION DATA

This application claims the benefit of EP Patent Application 06290547.6filed Mar. 31, 2006 entitled, “Cement Retarders” and also claims thebenefit of U.S. Provisional Application 60/746,806 filed May 9, 2006entitled “Cement Retarders”.

FIELD OF THE INVENTION

This invention relates to cement retarder systems and the retardedcement compositions produced by their use. In particular, the inventionrelates to such systems and compositions for use in cementing wells suchas oil, gas and water wells.

BACKGROUND OF THE INVENTION

Cement slurries are used in the construction of wells such as oil andgas wells. One such use is in the setting of casing in the well. In thisprocedure, a casing (typically a steel tubular liner) is positioned inthe well and a cement slurry is pumped from the surface down the casingso as to return to the surface in the annulus between the casing and theborehole wall. The cement fills the annulus and, once set, holds thecasing in place and provides fluid isolation between the zones throughwhich the borehole passes. In another use, a cement plug can be set in awell to prevent any further production of fluids to the surface.

Many wells can be several thousand metres in depth. This means that thetemperatures at the bottom of such wells may be significantly elevatedtemperatures, sometimes in excess of 250° C. (482° F.). This can lead toproblems in effective placement of the cement slurry. The time taken topump a cement slurry into a deep well can mean that the onset ofthickening caused by cement setting can become a problem, potentiallyleading to setting of the cement before it is properly placed eitheraround the casing or as a plug.

This setting phenomenon has lead to the development of a series ofadditives for the cement slurry known as ‘retarders’. These additivesact on the cement slurry to delay setting for a sufficient period oftime to allow the slurry to be properly placed. Examples of retardersfor use in borehole cement slurries can be found in U.S. Pat. No.5,503,671 and U.S. Pat. No. 5,503,672, the disclosures of which areincorporated herein by reference. These retarder systems are based onthe use of mixtures of borate salts (e.g. sodium tetraboratedecahydrate, boric acid, sodium pentaborate or potassium pentaborate)and ethylenediaminetetra (methylenephosphonic) (EDTMP) acid salts.However, these systems have been found difficult to use at temperaturesof 250° C. (482° F.) and above.

This invention seeks to provide a cement retarder system that iseffective at high temperatures, particularly when used with high densitycement slurries that are commonly used to plug deep, hot wells. Theinvention is based on the use of a copolymfr formed frp,(2-acrylamido-2-methylpropane-3-sulphonic acid) and acrylic acid and/oracrylamide. Such copolymers are known as fluid loss control additivesfor cement slurries, as disclosed in U.S. Pat. No. 6,277,900, thedisclosure of which is incorporated herein by reference.

SUMMARY OF THE INVENTION

The invention provides a cement retarder system, comprising a boratecompound,

an organophosphonate salt; and a copolymer formed from2-acrylamido-2-methylpropane-3-sulphonic acid (AMPS) and a monomerselected from the group consisting of acrylic acid, acrylamide andmixtures thereof wherein the components are present in a form thatallows mixing with a cement slurry.

In one embodiment, the borate-to-organophosphonate molar ratio is fromabout 8 to about 18, and preferably from about 12 to about 14.

In one embodiment the borate salt is selected from the group consistingof sodium pentaborate (Na₂B₁₀O₁₆), sodium tetraborate (Na₂B₄O₇) andboric acid (H₃BO₃). The borate salts can contain crystallization watermolecules.

In one embodiment, the organophosphonate is selected from the groupconsisting of the sodium or calcium salts of ethylenediaminetetra(methylenephosphonic acid) (EDTMP) hexamethylenediaminetetra(methylenephosphonic acid) and diethylenetriaminepenta(methylenephosphonic acid).

In one embodiment of the AMPS copolymer, acrylamide is used as themonomer.

In another embodiment of the AMPS copolymer, the monomer is acrylic acidor the copolymer is a terpolymer formed using both acrylic acid andacrylamide. The term ‘copolymer’ used here includes copolymers,terpolymers and mixtures and combinations thereof.

In one embodiment, the AMPS copolymer or terpolymer comprises from about40 weight percent (wt. %) to about 90 weight percent, preferably fromabout 60 wt. % to about 80 wt. %. The molecular weight of the AMPScopolymer is preferably from about 5,000 to about 2,000,000. In anotherembodiment, the AMPS copolymer has a molecular weight of from about600,000 to about 1,000,000.

In another embodiment, the copolymer is a combination of at least twocopolymers with different ranges of molecular weight. The molecularweight of the first of the two copolymers is typically from about 5,000to about 100,000 and the molecular weight range of the second copolymeris typically from about 600,000 to about 1,000,000.

The invention also provides a retarded cement composition comprising acement,

a liquid carrier, (typically water), a retarder system as defined abovepresent in an amount sufficient to delay setting of the cement; andoptional additives to otherwise modify the behaviour of the cementcomposition (e.g. antifoam agents, silica flour, bentonite, dispersantsand the like).

In one embodiment of the retarded cement composition, theborate/organophosphonate combination may be present in the retardedcement composition in amounts of from about 1 to about 12% by weight ofcement (BWOC).

In one embodiment, the copolymer is present in an amount of from about0.1 to about 0.6% BWOC.

In one embodiment, the cement is Portland cement (e.g. ISO/API class Gor H);

In another embodiment, the cement composition has a density greater than2.0 kg/L (16.7 lbm/gal).

The invention also provides a method of using a retarder system in anunderground well having a borehole drilled therein with a borehole wall,and further including a casing disposed in said borehole, such that anannulus exists between said casing and said borehole wall, said methodcomprising providing a cement composition and a liquid carrier,providing a retarder system for said cement composition comprising aborate compound, an organophosphonate salt, and a copolymer formed fromAMPS and a monomer selected from the group consisting of acrylic acid,acrylamide, and mixtures thereof, mixing a sufficient amount of saidretarder system to create a desired induction period with said cementcomposition and said liquid carrier to form a cement slurry, pumping thecement slurry from the surface down the casing, and causing said slurryto return to the surface in the annulus between the casing and theborehole wall.

In one embodiment, the method of using a retarder system in a cementingoperations in an underground well is performed when the temperature inthe well is greater than 150° C. (302° F.) and preferably greater than250° C. (482° F.).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a plot of heat flow vs. time for calorimeter measurementson various cement compositions.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows a plot of heat flow vs. time for a series of cementcompositions measured in an isothermal calorimeter to determine thelength of the induction period (time period during which the cementslurry does not set), the end of the induction period being indicated bythe sudden increase in heat flow. The concentration of borate wasmaintained constant at 4% BWOC (by weight of cement) in all testsplotted in FIG. 1, and the concentration of EDTMP was varied between 0%and 0.21%, as indicated. FIG. 1 shows that the length of inductionperiod varies with the borate: EDTMP ratio.

At 0% EDTMP the cement begins to set as soon as the test temperature isreached, (i.e., 182° C. [360° F.]), after about 2 and a half hours. Theaddition of very small concentrations of EDTMP (0.035 and 0.07% BWOC)significantly lengthens the induction period to 20 and 45 hours,respectively. Further additions of EDTMP lengthen the induction periodby a lesser amount, 7 and a half hours with 0.175% BWOC EDTMP. Withoutwishing to be bound by theory, it is believed that this reduction inretarding effect is due to the fact that EDTMP accelerates the hydrationrate of the ferrite phase (i.e., Ca₄Al₂Fe₂O₁₀) of Portland cements. Thisaccelerating effect is counterbalanced to some extent in the presence ofborate but the induction period is shortened when the borate-to-EDTMPratio becomes too low.

Adequate borate-to-EDTMP ratios allow long induction periods to beobtained when cement slurries are left under static conditions (i.e.,not agitated) as it is the case in the calorimeter cell.

The schedules of thickening time tests (consistometer) for examplesgiven below at different temperatures are shown in Table 1:

TABLE 1 BHCT ° C. Initial Pressure Final Pressure Time to reach (° F.)MPa (psi) MPa (psi) BHCT (min) 260 (500) 12.76 (1,850) 203.4 (29,500) 90274 (525) 12.76 (1,850) 203.4 (29,500) 95 288 (550) 12.76 (1,850) 203.4(29,500) 100

During the thickening time tests, the cement slurries are under dynamicconditions (i.e. agitated). In the consistometer there is a paddleinside the cell and the cell rotates at 150 rpm.

The effect of borate-to-EDTMP molar ratio on the thickening time ofcement slurries was investigated at bottomhole circulating temperature(BHCT) of 260° C. (500° F.), using sodium pentaborate decahydrate(Na₂B₁₀O₁₆.10H₂O) as the borate.

The basic cement slurry used is given below:

-   -   Class G Cement    -   2.7 L/tonne of cement of Antifoam Agent    -   35% BWOC Silica Flour    -   0.5% BWOC Bentonite    -   1.74% BWOC Dispersant (with 5% BWOC Pentaborate) or    -   2.32% BWOC Dispersant (with 10% BWOC Pentaborate)    -   5 or 10% BWOC sodium pentaborate decahydrate    -   0 to 0.89% BWOC EDTMP.

The solid additives are dry-blended with the cement, except thebentonite, which is pre-hydrated in the mix water and the antifoam agentis added to the mix water. The cement blends are mixed with fresh waterat a density of 1.89 kg/L (15.8 lbm/gal).

The results of consistometer (thickening time) tests for the variousmixtures are presented in Table 2 below:

TABLE 2 Pentaborate- Thickening Time @ Pentaborate EDTMP to-EDTMP 260°C. (500° F.) (% BWOC) (% BWOC) Molar Ratio (hr:min) 5 0 — 1:39 5 0.1524.5 4:17 5 0.30 12.4 4:46 5 0.44 8.4 5:19 10 0.30 24.5 1:58 10 0.5912.4 5:30 10 0.89 8.3 4:33

The results demonstrate a synergy between the two components, i.e.,between pentaborate and EDTMP.

It can be shown that there is a synergy between borate/EDTMP retardersand AMPS-acrylamide fluid loss control additives, especially at BHCTsabove 149° C. (300° F.). This synergy results in the need for a lowerretarder concentration to achieve a given thickening time at a givenBHCT. It is believed that the acrylamide part of the copolymer canhydrolyze into acrylate, which is known to be able to retard thehydration of cements.

The effect of AMPS-acrylamide fluid loss control additives on thethickening time of cement slurries retarded with 10% BWOC sodiumpentaborate decahydrate and 0.59% BWOC EDTMP (Pentaborate-to-EDTMP molarratio of 12.4) is shown in Table 3 below for the following cementcomposition:

-   Class G Cement-   2.7 L/tonne of cement Antifoam Agent-   35% BWOC Silica Flour-   2.32% BWOC Dispersant-   10% BWOC Sodium Pentaborate Decahydrate-   0.59% BWOC EDTMP-   0 to 0.3% BWOC AMPS-acrylamide fluid loss control additive.

The solid additives are dry-blended with the cement. The antifoam agentis added to the mix water. The cement blends are mixed with fresh waterat a density of 1.89 kg/L (15.8 lbm/gal).

TABLE 3 AMPS-acrylamide fluid loss Thickening Time @ 260° C. controladditive (% BWOC) (500° F.) (hr:min) 0 5:30 0.1 7:19 0.2 9:10 0.3 6:31

The addition of small amounts of AMPS-acrylamide fluid loss controladditive lengthens the thickening time.

So the presence of AMPS-acrylamide fluid loss control additive can bebeneficial to obtain longer thickening times at elevated temperatures.However, too high concentrations of AMPS-acrylamide fluid loss controladditive may be detrimental. It can be shown that an optimumconcentration of AMPS-acrylamide fluid loss control additive exists toensure longer thickening times at elevated temperatures. The optimumconcentration varying between 0.1% and 0.6% BWOC may depend on severalparameters such as the cement brand, pentaborate and EDTMPconcentrations, temperature, presence of other additives and theirconcentrations. For example, for cement of Table 3, optimumconcentration will be at 0.2% BWOC and for cement of Table 6 (Tests7-8), optimum concentration will be at 0.4% BWOC.

The density of the cement slurries can be controlled using thetechniques described in EP 0621247, incorporated herein by reference.This approach is used to increase the density of cement slurries used inthese tests. Such high density cement slurries are prepared using eithera Class G cement or a Class H cement and the effect of AMPS-acrylamidefluid loss control additive and borate-to-EDTMP molar ratio on thethickening time is studied.

First Series of Experiments

For the first series of experiments, a high density cement blend, madeof cement class G, is used with the following additives:

-   -   2.5 L/tonne of blend of Antifoam Agent    -   1.47% BWOC Dispersant    -   7.2% BWOC Sodium Pentaborate Decahydrate    -   0.42% BWOC EDTMP    -   0 to 0.27% BWOC AMPS-acrylamide fluid loss control additive.

The solid additives are dry-blended with the high density cementmixture, which is mixed with fresh water at a density of 2.16 kg/L (18lbm/gal). The antifoam agent is added to the mix water.

The solid volume fraction (SVF, percentage of total slurry volume thatis provided by solid components) is 60%.

The Pentaborate-to-EDTMP molar ratio is maintained at 12.7, and theconcentration of AMPS-acrylamide fluid loss control additive varied.

Thickening times are measured at 274° C. (525° F.) and the resultsgathered in Table 4, demonstrating that small concentrations ofAMPS-acrylamide fluid loss control additive can provide significantincrease in thickening time. These results are in agreement with thedata shown in Table 3.

TABLE 4 AMPS-acrylamide Thick. Time @ fluid loss Pentaborate- 274° C.Pentaborate EDTMP control additive to-EDTMP BHCT (525° F.) Test % BWOC %BWOC % BWOC Molar Ratio ° C. (° F.) (hr:min) 1 7.2 0.42 0 12.7 274 (525)3:12 2 7.2 0.42 0.16 12.7 274 (525) 3:57 3 7.2 0.42 0.27 12.7 274 (525)5:11Second Series of Experiments

For the second series of experiments, a high density cement blend, madeof cement class G, is used with the following additives:

-   -   2.5 L/tonne of blend of Antifoam Agent    -   2.23% BWOC Dispersant    -   8.9% BWOC Sodium Pentaborate Decahydrate    -   0.3% BWOC AMPS-acrylamide fluid loss control additive    -   0.52 to 0.69% BWOC EDTMP.

The high density cement mixture is prepared as in the first series ofexperiments to give a slurry of density 2.16 kg/L (18 lbm/gal) and SVFof 60%.

The concentration of AMPS-acrylamide fluid loss control additive ismaintained constant at 0.3% BWOC, and the concentration of EDTMP varied(Tests 4 and 5 in Table 5 below). Thickening times are measured at 274°C. (525° F.). Thickening time decreases with increased EDTMPconcentration (i.e., lower Pentaborate-to-EDTMP molar ratio). This is inagreement with data shown in Table 2 when 10% BWOC pentaborate is used.A thickening time of about 4 and a half hours can be achieved at 288° C.(550° F.) with a Pentaborate-to-EDTMP molar ratio of 12.6 and when 0.3%BWOC AMPS-acrylamide fluid loss control additive is present in theslurry (Test 6).

TABLE 5 AMPS-acrylamide fluid loss Pentaborate- Thick. Pentaborate EDTMPcontrol additive to-EDTMP BHCT Time Test % BWOC % BWOC % BWOC MolarRatio ° C. (° F.) (hr:min) 4 8.9 0.52 0.3 12.6 274 (525) 6:47 5 8.9 0.690.3 9.5 274 (525) 5:48 6 8.9 0.52 0.3 12.6 288 (550) 4:36Third Series of Experiments

For Tests 7 and 8 in Table 6 below, a high density cement blend, made ofcement class H, with following additives is used:

-   -   2.5 L/tonne of blend of Antifoam Agent    -   1.49% BWOC Dispersant    -   8.1% BWOC Sodium Pentaborate Decahydrate    -   0.47% BWOC EDTMP    -   0.27 to 0.41% BWOC AMPS-acrylamide fluid loss control additive

The high density cement mixture is prepared as in the first series ofexperiments to give a slurry of density 2.16 kg/L (18.0 lbm/gal) and SVFof 60%.

For Tests 9 and 10 in Table 6 below, a high density cement blend, madeof cement class H, with the following additives is used:

-   -   2.5 L/tonne of blend of Antifoam Agent    -   1.63% BWOC Dispersant    -   8.9% BWOC Sodium Pentaborate Decahydrate    -   0.52% BWOC EDTMP    -   0.59% BWOC AMPS-acrylamide fluid loss control additive.

The high density cement mixture is prepared as in the first series ofexperiments to give a slurry of density 2.16 kg/L (18.0 lbm/gal) and SVFof 60%.

Tests 7 and 8: As previously observed with Class G Cement (see Table 4),the thickening time is lengthened when increasing the concentration ofAMPS-acrylamide fluid loss control additive.

Tests 9 and 10: Quite long thickening times can be achieved at 274° C.(525° F.) and 288° C. (550° F.) when the Pentaborate-to-EDTMP molarratio is 12.6 and when 0.59% BWOC AMPS-acrylamide fluid loss controladditive is present in the slurry.

TABLE 6 AMPS-acrylamide fluid loss Pentaborate- Thick. Pentaborate EDTMPcontrol additive to-EDTMP BHCT Time Test % BWOC % BWOC % BWOC MolarRatio ° C. (° F.) (hr:min) 7 8.1 0.47 0.27 12.7 274 (525) 1:16 8 8.10.47 0.41 12.7 274 (525) 4:26 9 8.9 0.52 0.59 12.6 274 (525) 7:49 10 8.90.52 0.59 12.6 288 (550) 5:23Fourth Series of Experiments

For the fourth series of experiments, a high density cement blend, madeof cement class H, with the following additives is used:

-   -   2.5 L/tonne of blend of Antifoam Agent    -   1.63% BWOC Dispersant    -   8.9% BWOC Sodium Pentaborate Decahydrate    -   0.45% BWOC AMPS-acrylamide fluid loss control additive    -   0.42 to 0.62% BWOC EDTMP

The high density cement mixture is prepared as in the first series ofexperiments to give a slurry of density 2.16 kg/L (18.0 lbm/gal) and SVFof 60%. Results are shown in Table 7.

The thickening time decreases with increased EDTMP concentration. Theseresults are in agreement with those obtained with Class G cement (seeTests 4 and 5 in Table 5). The thickening time of Test 12 is shorterthan that of Test 9 (see Table 6), which contains a higher concentrationof AMPS-acrylamide fluid loss control additive. This is in agreementwith the Tests 1, 2 and 3 in Table 4, and with the Tests 7 and 8 inTable 6.

TABLE 7 AMPS-acrylamide fluid loss Pentaborate- Thick. Pentaborate EDTMPcontrol additive to-EDTMP BHCT Time Test % BWOC % BWOC % BWOC MolarRatio ° C. (° F.) (hr:min) 11 8.9 0.42 0.45 15.6 274 (525) 7:00 12 8.90.52 0.45 12.6 274 (525) 6:02 13 8.9 0.62 0.45 10.6 274 (525) 5:11

1. A retarded cement composition comprising cement, a liquid carrier anda retarder system comprising a borate compound, an organophosphonatesalt, and a copolymer formed from and a monomer and a monomer selectedfrom the group consisting of acrylic acid, acrylamide and mixturesthereof, said retarder system present in an amount sufficient to allowmixing with the cement and to delay setting of the cement.
 2. The cementcomposition of claim 1, wherein the retarder system is present in anamount such that the borate/organophosphonate combination is present inan amount of from 1 to 12% BWOC (by weight of cement).
 3. The cementcomposition of claim 1, wherein the retarder system is present in anamount such that the copolymer is present in an amount of from 0.1 to0.6% BWOC.
 4. The cement composition of claim 1, wherein the density ofthe composition is greater than 2.0 kg/L (16.7 lbm/gal).
 5. The cementcomposition of claim 1 wherein the AMPS copolymer has a molecular weightis between 5,000 and 2,000,000.